Okay, I am probably crazy, but I am trying to convert my outdoor central boiler to burn coal. I have installed cast iron radiater as grate , elevated for bottom air flow and ash clean up and a shaker attachment. I have 200# coal inside with a red hot center, blue flame coming off the top about 2-3" and my water jacket which is basically above the fire and will not get the water above 130 - what gives???? The coal pile is side to side and 8 -10 " deep.

You need to have all the air entering the stove from below the grate, and none from over the fire. Coal burns from the bottom up. If you have any over fire draft you need to close that off. Also, you may not have enough air flow into the boiler under the grate. Coal burning is very different then wood burning. Almost any stove will burn wood, coal requires very specific conditions.

I have elevated the radiators as grates above the air entry. It does seem to be burning properly from the bottom up, blue flame on top etc.. The center is red hot and the ashes are dropping down nicely, but the water isn't getting hot enough even though I have added a maual damper for the flue.

Is the "air channel" pretty tight around the fire area and the heat exchanger area? In other words, "must the air go through the coal or can it skirt around the perimeter of the coal?" and same with exchanger. Do you feel that there is a large enough area of "hot coals"? (You've mentioned "hot in the center") Does the outer area get hot and burn?

My guess is most of your heat is going out of the chimney or being lost elsewhere, the flue pipe for example on a typical stoker will be hot to the touch but not enough to burn your hand unless you left it there. The exhaust pip exits anywhere from the bottom to half way up the units containing the heat in the top of the stove/furnace.

EFM for example makes a oil add on for coal furnaces, at least they did. I don't what the EFM guy will tell you but I'd imagine it's going to be the oil is for emergency use and is not very efficient. Vice versa a boiler made to burn oil isn't going to be a very economical way to burn coal. It's a different design and setup.

There is a 6" space in front of the coal supply and I will bet the air is traveling up and overthe burning coal acting as both a jet stream and an insulator to the water jacket wiskig the hot air out the flue. Reading these comments has been great. I can change that system to eliminate the air flow up the face of the fuel and force it to go up through it. Can't wait to get home and try it.

Dallas wrote:Is the "air channel" pretty tight around the fire area and the heat exchanger area? In other words, "must the air go through the coal or can it skirt around the perimeter of the coal?" and same with exchanger. Do you feel that there is a large enough area of "hot coals"? (You've mentioned "hot in the center") Does the outer area get hot and burn?

Last night I started my stove from cold with cedar shingle kindling, charcoal, then coal on top. After I had a good hot fire going, and plenty of heat coming off the stove, I noticed I had a piece of shingle left over, so I broke it up and tossed it in on top of the coal. The top layer of coal was not yet burning red. Well! The shingle just sat there for fifteen minutes before it even started to burn. So, the space over the fire where my shingle was, and where your water jacket is, apparently is NOT an especially hot spot, especially if there is no glowing coal there to radiate heat. Fooled me and fooled you too.

Be sure to post results of your next modification. Surface area of the water jacket, and contact time between the gases and the jacket, would seem to be important. However, I wonder how much of the heat is radiant, from the glowing coal rather than in the gases. Seems like exposure to the infrared radiation could also be a critical factor in jacket design and placement.

Water jacket is 18" above the fire, so that means there is an air space above the coal that is 18 x 36 x48. The coal is burning to within 4" of the edges red hot and about 10" deep the same 36 x 48. The air all comes from under the coal via the slots in a large radiator I used with a flat cast iron grate the same size with an open pattern creating 1" holes over the entire grate. There is no air flow on either side or behind the coal and I just fitted a sheet metal piece along the front so that the air is no longer going up and over the coal. The flue is connected by an 8" x 10" channel that I put a damper on that can limit the air flow out to a 3/4 x 10 slot to fully open. The fire seems to be burning fine . I have had it going solidly fo 10 days. I add 75 - 100# 2 times per day. The ash drops down and I can shovel it out fine. I use a long rod to "shake the coal" and keep the slots from clogging. This all seems to work other than my water needs to be 160+

If you kept it going for a week then it must be burning OK. My first thought is as the leader said, its going up the chimney. Barometric damper installed? What is the temperature of the flue/stack gases? Do you have a blower or is it natural draft? How much water is in this system and will it pick up heat with the circulating pump off? My water jacket at the crown is 21" or more over the fire, but it does come down on the sides to the top of the firebrick. Last but not least, does this thing have a water jacket that goes under the firebox?Something isn't right, with a 36" X 48" grate you should be able to melt steel, never mind get water over 130*. Does it have a by-pass damper? That will double the residence time of the hot gases.

You said this is an outdoor boiler. Where are you measuring the water temperature -- how far from the outlet of the water jacket? Is the pipe in between well insulated? What's the temperature of the outside of the unit? Is it also well insulated or are you losing a lot of heat into the great outdoors? (Or up the chimney, as others have wondered?)

I'm also curious about your 8 x 10 opening to the flue -- 80 square inches maximum, but you didn't say how much you damp it down. The six inch stovepipe for my much much smaller Harman Mk I is roughly 29 or 30 square inches, if I remember my high school math. And you have 12 square feet of coal times 10 inches deep, where I have perhaps 1 1/2 square feet times ten inches deep -- only 1/8 the cubic content. So what have we proved? Darned if I know! The flue sizes don't seem to be in the ratio I would expect, though I have no idea what the formulas are for proper flue size. And as coaledsweat says, if you have kept it burning for a week you must be doing something right. I'm kind of rambling here, just sort of thinking out loud, but maybe it will trigger an idea....

Thank you cArNaGe for posting the cut away. Using fire brick I elevated a cast iron radiater off of the floor creating an ash cleanout area and air supply tunnel under the fire. The top of that is just above the air intake at the door.The air supply is controlled by a capacitor that shuts off the air supply when the water gets to 180. I have a steel plate across the top of that to stop the air from going up the face of the coal and above the fire. The top of the coal pile is kept at about the bottom of that #9 water flange dropping down creating a heated chamber of air above it. It all makes common sense to me and the fire burns great other than the water, as measured in the water jacket, stays 110 -120's and yes I could melt steel in this fire that is burning. I have a manual damper on the flue and do close that down to keep the heat in the stove longer. Nothing I have done is air tight or even very scientific, but it burns coal well. Heating my house was more my plan. I may have to go back to wood if it gets cold. I can place my arm in the firebox and still have hair on my arm even though the fire is hot. The air does not seem as hot as it does when I have a wood fire in that box. The wood fire heats the house fine, but I am loading a cord of wood in this thing in 5 days. I love the no smoke of coal and find it easy to work with. I want to thank all of you for adding to this tale I have started. I relly want to figure this out, I am so close.

You put a cord of wood through in 5 days, and perhaps 750 to 1000 pounds of coal in the same time. We can discuss efficiencies and what kind of wood and how wet is the wood, but my very-rough calculations say you are getting at least 12 million BTU delivered from the wood, and maybe 7 to 9 MBTU from the coal depending whether it's 150 or 200 pounds a day. So perhaps that accounts for some of the difference between 130 and 180 degree water. Can you get it to burn more coal, faster?

And I'm still wondering about the radiant heat of coal, versus the quantity of hot gases from wood. Burning wood last year, and coal this year, my stack temperature at high heat output was considerably higher with wood, same chimney and same stove pipes. And my bare arm will get baked a lot quicker from the coal's radiant heat compared to the wood's. Maybe your heat exchanger works better with a lot of hot gas passing through it. Of course that would leave the question of where all that radiant heat is disappearing to, so maybe I'm all wet on the radiation thing. But I think I'm on solid ground with the BTU calculations.

Your are probably going to have to make sure there is no air bleeding around the outside of the fire and put a blower on it to force a draft. It will burn more coal, but I would think it could pull the temps up 60-80*. One problem is that thing has an enormous water volume and you will be chasing its loses. The water under the firebox is not helping either. Your going to have to burn close to 1500# in 5 days to match the woods output.